In the hot-gas path of gas turbines, nickel-based and cobalt-based materials are used. Owing to their optimization to the highest possible strength, however, these materials often do not have sufficient resistance to oxidation and high-temperature corrosion in the hot gas. Therefore, the materials have to be protected from attack by the hot gas using suitable protective coatings. To increase the turbine inlet temperature, a ceramic layer based on zirconium oxide is also additionally applied to components subject to extremely high thermal stresses for thermal insulation. The realization of the highest possible operating temperatures and a long service life of the components which are exposed to hot gas requires an optimally adapted protective layer system consisting of a bonding layer and a thermal barrier coating. The composition of the bonding layer here is of central importance.
To solve this problem, protective layers are applied to the hottest components in part also as a bonding layer for a thermal barrier coating. These generally consist of what are known as NiCoCrAlY covering layers, which, in addition to nickel and/or cobalt, can also contain chromium, aluminum, silicon, rhenium, tantalum and rare earth elements such as yttrium, hafnium and the like. However, further increasing surface temperatures on the protective layer can lead to damage, which results in failure of the layer or in spalling of the thermal barrier coating. Rhenium has often been used.
However, rhenium has the disadvantage that its content considerably increases the costs. This has been particularly significant in the past few years and will also play a major role in the future.
Given increasing temperatures of the layer surface or for longer service lives of the protective layers, it is necessary to develop suitable protective layers which, under these boundary conditions, have improved oxidation resistance combined with a sufficiently good thermomechanical resistance and at the same time lower costs than rhenium-containing layers. This can be achieved only by a very balanced chemical composition of the protective layer. Here, the elements Ni, Co, Cr, Al and Y are particularly important.
The fact that these elements also interact with the base material owing to diffusion must also be taken into great consideration.